1. Technical Field
This invention relates to the measurement of chemical and physical properties of flowing streams of liquids, solids or mixed liquids and solids by quantitative nuclear magnetic resonance (NMR) spectroscopy, but may be applied to other types of molecular spectroscopy including infrared (IR), near-infrared (NIR), and Raman spectroscopy. In particular, this invention relates to sample analyzers and systems for managing sample introduction into the analyzers.
2. Background Information
The analysis of materials using NMR requires a region of space containing a magnetic field that is either extremely uniform in magnetic flux density or else extremely uniform in the spatial gradient of magnetic flux density. In such a region, a sample to be analyzed is subjected to a short pulse of electromagnetic energy at a predetermined frequency that is a function of the atomic nuclei to be analyzed and of their chemical bonding. The pulse is coupled to the sample by a surface coil. A typical pulse duration is of the order of fifty microseconds, although the pulse width that is chosen is a function of the characteristic relaxation time of the subject nuclei in material being analyzed. The magnetic field causes the magnetic moments of the constituent nuclei in the sample to become aligned along lines of magnetic flux. If the field is strong and uniform to a relatively high degree of precision, the magnetic moments will be essentially parallel to each other, resulting in an aggregate or bulk magnetic moment. The electromagnetic energy coupled to the sample effects a change in the bulk magnetic moment. The relaxation of the bulk magnetic moment from the re-aligned position back to the original position when the pulse is ended produces signals that may be detected and transformed into a spectrum, in which response intensity plotted as a function of frequency is unique to the particular sample composition. Due to the temperature dependence of both the consistency of the magnetic field and a sample's spectral behavior, temperature control of both within appropriate tolerances is desired.
NMR spectrometers came into use in the research laboratory in the early 1960s. As NMR technology advanced, it became a ubiquitous tool for elucidating molecular structure, chemical behavior and reaction mechanisms, and molecular level interactions in biological systems and diverse organic molecules including pharmaceuticals and polymers. Generally such investigations are performed on pure materials or carefully controlled mixtures to permit observation of the chemical phenomenon of interest. Understanding about the chemistry may be deduced through examination by a skilled spectroscopist of features in a single spectrum or a series of spectra measured on the subject chemical system under limited number of experimentally controlled conditions.
More recently, NMR analysis has also been used in various production environments to analyze flowing liquids, pastes, slurries, or solids in powdered or other finely divided form. NMR analyzers may be used to perform in minutes the measurement of multiple chemical and physical properties, which otherwise would need to be analyzed by many different analytical methods in a quality control laboratory, requiring hours or even days. This capability to perform multiple analyses with high frequency automatically, makes process NMR analysis a cost-effective means for characterizing input and output streams associated with diverse chemical processes. A typical NMR analyzer commonly used for such process applications includes the D-Mash NMR analyzer available from The Qualion Company, Haifa Israel.
Process streams that may be difficult to analyze continuously by means other than NMR include those which have high optical density, high loading of fine particulates or solids, high water content, or relatively high viscosity. Exemplary applications therefore may include those in oil and petrochemical processing plants, chemical plants, and other industries requiring automatic process control of fluids. In the petrochemical field, these analyzers have been used in crude blending, fast CDU optimization after crude feed switching, effective feed control and optimization of FCC (Fluid Catalytic Cracking) unit applications. Other industries that may benefit from NMR analyzers include plastics and polymers, pharmaceuticals, food and beverages.
In contrast to traditional use of analytical NMR spectroscopy in research applications, petrochemical streams can contain dozens or hundreds of compounds. Accordingly, the objective is no longer structural elucidation or detailed chemical characterization, but the measurement of bulk properties such as distillation yields at various temperatures, the total aromatic content, acidity, bulk sulfur content, octane in gasoline, cetane in diesel, and cloud point in jet fuel. Also, the analysis of a sample spectrum to obtain property values is performed automatically by software, which applies property-specific models based on data sets of dozens or hundreds of calibration samples whose spectra were measured previously and corresponding property values analyzed in the laboratory.
It follows that the quality of property models depends at a minimum on the compositional diversity of the calibration sample set and the precise measurement of NMR spectra. The latter requires proper tuning of the NMR spectrometer to achieve a magnetic field of suitable uniformity and the supply to it of samples maintained at relatively uniform temperature. For example, even relatively slight variations in sample temperature, e.g., variations as small as plus or minus 3 degrees C., may significantly reduce measurement accuracy. A set of calibration samples whose compositional diversity is suitable for creation of property models may be obtained most conveniently by collecting and storing samples over a relatively long time frame during which feed or product streams associated with a process exhibit relevant property variations.
A need therefore exists for an apparatus and method for maintaining and supplying samples to an NMR analyzer in a relatively uniform state, both on-line within a process, and off-line in a batch processing mode.